Electrophoresis of model colloidal spheres in low salt aqueous suspension

We report on comprehensive measurements of the electrophoretic mobility mu of a highly charged spherical colloid in deionized or low salt aqueous suspensions, where fluid and crystalline order develops with increased packing fraction Phi. We propose the existence of a 'universal' shape of the mu(Phi) showing three distinct regimes: a logarithmic increase, a plateau and a logarithmic decrease. The position and the height of the plateau are found to be influenced by the particle surface properties and the electrolyte concentration. In particular, it starts once the counter-ion concentration becomes equal to the concentration of background electrolyte. This coincides only loosely with the range of Phi where fluid order is developing. Also the better defined first order freezing transition is observed to be uncorrelated to the shape of the mu(Phi) curve.